Combustion derived single phase Y4Al2O9:Tb3+ nanophosphor: crystal chemistry and optical analysis for solid state lighting applications

Cool green light emanating monoclinic Y4−xAl2O9:xTb3+ (x = 1–5 mol%) nanophosphors have been fabricated through gel-combustion method. X-ray diffraction and transmission electron-microscopy data have been utilized to assess their structural and microstructural characteristics, including cell parameters and crystallite size. Uneven aggregation of nanoparticles in the nano-scale with distinctive porosity can be seen in the TEM micrograph. Kubelka–Munk model imitative diffuse reflectance spectra and an optical band gap of 5.67 eV for the Y3.97Al2O9:0.03Tb3+ nanophosphor revealed high optical quality in the samples, which were thought to be non-conducting. The emission (PL) and excitation (PLE) spectra as well as lifetime measurements have been used to determine the luminescence characteristics of the synthesized nanophosphors. The emission spectra show two color i.e. blue color due to 5D3 → 7FJ (J = 4 and 5) transitions and green color due to 5D4 → 7FJ (J = 3, 4, 5 and 6) transitions. The most dominant transition (5D4 → 7F5) at 548 nm was responsible for the greenish color in focused nanocrystalline samples. Calculated colorimetric characteristics such as CIE, and CCT along with color purity of the synthesized nanocrystalline materials make them the best candidate for the solid-state lighting (SSL).


Introduction
Development of the upcoming generation of highly power efficient, sustainably manufactured solid state lighting (SSL) devices is a primary concern for scientists and researchers globally, especially material scientists, as the world is now experiencing an energy crisis. [1][2][3][4][5] The signicant quantity of power wasted in our modern world is due to articial lights. Solid state illumination technology has completely revolutionized the eld of luminance due to its exceptional properties, which include high energy efficacy, superior optical assets, excellent lumen performance, and environmental stewardship. [6][7][8][9][10][11][12] It is designed to meet the global demand for both indoor and outdoor lighting. Moreover, host matrixes based on aluminates that have enhanced luminance efficiency, excellent CCT and CRI (color rendering index), superior crystallinity, simple synthesis, superior mechanical-strength, signicantly greater chemical and thermal stability, temperature resistance, etc., are extremely desirable for doping with an appropriate dopant ion. [13][14][15][16][17][18][19][20] For solid state lighting and display, yttrium and aluminium host nanophosphors are particularly benecial. Yttrium aluminium perovskite (Y/Al; 1 : 1) is utilised as a scintillator, and rare earth (RE) activated yttrium aluminium garnet (Y/Al; 0.6 : 1) is frequently employed as a host material for laser action. [21][22][23][24] Nevertheless, there have been very few reports of spectroscopic studies on Y 4 Al 2 O 9 (Y/Al; 2 : 1) doped with rare earth ions (RE 3+ ). The crystal structure of Yttrium aluminium oxide (Y 4 Al 2 O 9 ) abbreviated as YAM corresponds to monoclinic system with space-group P2 1 /c. 25 The Y atoms are coordinated to either six or seven oxygen atoms with site symmetry C1. The Al atoms are present in four coordination environment with oxygen atoms. 26 Compared to Y 3 Al 5 O 12 and YAlO 3 systems, the YAM host has a lattice structure that allows four unique places where trivalent rare earth (RE 3+ ) ions might be replaced. Only a few articles have described the spectroscopic characteristics of the RE 3+ doped YAM crystal revealing its multi-tenant characteristics. Due to the partially lled 4f-subshell in the RE ions, which is shielded by the surrounding 5s and 5p shells, rare-earth (RE) ions function as dopant in phosphors nanocrystalline materials. [27][28][29] The remarkable luminous properties of rare earth activated phosphors are caused by the emission (PL) spectra of produced f-f transitions, which typically produce luminous, precise peaks in visible-range (400-800 nm) of spectrum. Trivalent terbium (Tb 3+ ) ion is known to be a vivid green emitter among the trivalent lanthanide ions due to its distinctive 5 D 4 / 7 F 5 transition. 30 It is a remarkable host activator having great applicability in SSL and display technology. The existing research indicated that the coprecipitation approach, traditional solid state reaction method and Pechini method were oen used to generate the rare earth doped Y 4 Al 2 O 9 nanomaterials. [31][32][33] But, these synthetic methods suffer from numerous connes like non-homogeneous nature, necessity of higher-temperature, more consumption of time etc. 34 In context of current study, low temperature based gel combustion (GC) method was employed to synthesized Y 4−x Al 2 O 9 :xTb 3+ (x = 1-5 mol%) crystalline materials and a systematically investigation about their structural and photophysical characteristics have been done. Xray diffraction was utilised to investigate the crystal structure. The functional groups found in host were examined using Fourier transform-infrared (FTIR) spectroscopy. Morphology of the material is analyzed by TEM study. The comparative percentage of integral atoms in fabricated nanomaterials is studied via energy-dispersive X-ray (EDX) exploration. Luminescent parameters revealed that the synthesized materials are appropriate candidate for SSL applications.

Materials used
.97 Al 2 O 9 :0.03Tb 3+ and a series of Y 4 Al 2 O 9 :Tb 3+ nanophosphors with diverse concentration of Tb 3+ cation (1-5 mol%) have been synthesized via solid state reaction and gel-combustion method respectively. For preparation purpose, urea has been utilized as fuel. The chemicals viz., nitrates of yttrium, terbium and aluminium was used for preparation purpose, purchased from Sigma-Aldrich. Deionized water was used as solvent.

Synthesis of bulk materials
To prepare the bulk materials, stoichiometric amounts of initial chemicals were taken without any further purication. All of them were mixed and grinded uniformly for 1 h using mortar & pestle. Grinded mixture was then kept in an alumina crucible and sintered for 4 h at 1100°C in a box furnace. Finally calcined material was ground thoroughly again to get the resultant ne bulk phosphor for different analysis.

Synthesis of nanophosphors
Firstly, starting materials were taken in desired stoichiometric proportion and dissolved completely in water. Upon heating at ∼80°C, this prepared mixture was become viscous have gel type appearance due to vaporization of solvent (H 2 O) molecules. Then, a considered quantity of fuel with deionized (DI) water has been added to resultant solution and further allowed to heat. The formed gel type mixture was allowed to combust for een minutes in a pre-heated furnace at 600°C. In this single step preparation technique, gaseous products viz., oxides of carbon and nitrogen expelled out during combustion. 35,36 The low reaction temperature is answerable for formation of nano-crystalline materials. This combustion technique is exothermic in nature which results in high crystallinity of synthesized materials. The porous product was allowed to cool with subsequent grinding to obtain ne powdered material. The formed powdered sample was further calcined at 800°C to obtain desired Y 4 Al 2 O 9 :Tb 3+ nanophosphors.

Instrumentations
The crystalline phases were examined on a "Rigaku Ultima X-ray diffractometer" (operating potential: 40 kV & current: 40 mA) and mono-chromatic radiations were produced from Cu-anode tube linked with Johansson-monochromator for Ka 1 (l = 1.54059 Å). The Bragg-Brentano geometry was obtained in range of 2q from 10°to 80°with a step interval of 0.02°and scan pace of 2°min −1 . For undoped and doped samples, Rietveld analysis was performed to obtain crystallographic and renement parameters. The vibrational spectra were obtained on "PerkinElmer 5700 FTIR spectrometer" in solid state by using pellets of anhydrous potassium bromide. To analyze the content of constituent elements, the EDX patterns of prepared nano-samples were recorded. To examine the structural analysis was done through transmission electron microscope (JEOL JEM-1400 Plus). For obtaining optical band gap data of phosphors, their respective reectance spectra were recorded on a UV-3600 Plus, Shimadzu UV-Vis-NIR spectrophotometer (DRS). Photoluminescence spectral proles and quantum yield were recorded using a Horiba Jobin YVON Fluorolog Model FL-3-11 equipped a 150 W pulsed xenon lamp. The luminescence lifetime has been noted on a Hitachi F-7000 FL spectrophotometer.

XRD evaluation
XRD Patterns of synthesized bulk and nanophosphors have been extensively explored to determine crystalline nature and phase identication. Fig. 1 demonstrates the diffraction patterns of bulk materials such as host YAM and Y 3.97 Al 2 O 9 :0.03Tb 3+ materials. All of the peaks are well matched with the JCPDS card 46-0396. 37 There is no additional impurity peak in the XRD pattern, conrming single phase of bulk materials and also monoclinic structure having space group P2/ c. High intensity of the XRD peaks denes their high degree of crystallinity as compare to the nanophosphors which was the result of high sintering temperature during the solid state reaction route. The average crystallite size (D) of the host and Y 3.97 Al 2 O 9 :0.03Tb 3+ bulk materials were found to be 127.02 nm and 122.66 nm respectively, which is much higher compared to the nanophosphors. Fig. 2(a) depicts the diffraction spectra of undoped Y 4 Al 2 O 9 and different concentration (1-5 mol%) of Tb 3+ doped Y 4 Al 2 O 9 nanophosphors. All synthetic materials are crystallized, as seen by the diffraction patterns. The diffraction proles of the prepared nanosamples are well accorded with standard JCPDS number 46-0396, 37 which affirm existence of monoclinic-crystal phase with P2 1 /c space-group. 38 Aer addition of dopant ion with different concentration in the host materials, there is no additional peaks were noticed in the XRD spectral proles which conrm the pure synthesis of the Y 4−x -Al 2 O 9 :xTb 3+ (x = 1-5 mol%) nanophosphors. However, some alternation in the peaks position is observed in the XRD lines of the doped samples, which was the evidence of Tb 3+ content in the prepared doped samples. The XRD peaks were shied in the lower angle side with the incorporation of dopant ion concentration, this was due to replacement of smaller size Y 3+ ion by the of bigger size Tb 3+ ion in the host material, as demonstrated in Fig. 2(b). Due to substitution of smaller host ion by larger size activator ion, it was noted that the estimated interspacing dvalues increased with concentration of activator ions as summarized in the Table 1. It is validate with the Bragg's relation (2d sin q = nl) that to remain nl constant, 2q-angle would be decreased, which resembles with the shiing of peaks in the lower angle side. 39 There are two replicating sites of host material (Y 3+ and Al 3+ ) are available for the activator ion (Tb 3+ ) because of their same charge. To assess the viability of replacement, the percentage differences between the dopant and host ions should not be exceed than 30%. 40 Relation (1) given below is exploited for assessment percentage radii difference between the host and dopant ions. 41 represent ionic-radii of host and incoming cation alongwith coordination number, separately. Computed D r -value has been found to be less than 30%, which demonstrates successful substitution of Y 3+ by Tb 3+ ion. The intensity prole is assessed using the Rietveld renement method, which enables an approximate model with a real foundation for crystal and renement characteristics of synthesized materials. Fig. 3 Here, D hkl symbolizes crystallite size, l used for X-ray wavelength, b denotes width of pure diffraction patterns in radians, and K becomes constant (0.89). The conceivable estimation of the crystallite size for Y 4 Al 2 O 9 (YAM) and Y 4−x Al 2 O 9 :xTb 3+ (x = 1-5 mol%) nanomaterials was around 37-42 nm. Also, the existence of microstrain in crystalline material results into peak shiing and line broadening in diffraction patterns. Hence, using W-H linear tting method, the strain present in the considered nanosamples was assessed by using eqn (3). 45 Here, b hkl is peak broadening and D & l are crystallite size and Xrays wavelength respectively. W-H linear tted graph for undoped YAM and YAM:xTb 3+ (x = 1-5 mol%) in Fig. 5 owing straight line graph between 4 sin q hkl (on x-axis) and b hkl cos q hkl (on y-axis). The crystallite-size and induced microstrain were determined from values of intercept and slope separately. The computed values are tabulated in Table 3.

SEM and TEM examination
SEM image of Y 3.97 Al 2 O 9 :0.03Tb 3+ nanopowder is shown in Fig. 6(a). According to the SEM prole, particles in the optimized sample have a non-uniform shape and lie in micron-size. This might be as a result of the agglomeration caused by the combustion of material, which releases gas byproducts during  the combustion. To comprehend the morphological characteristics (shape & size) of crystalline materials, TEM exploration has been used. TEM micrograph of materials that were re-heated at 800°C is shown in Fig. 6(b). The shape of the particles found to be non-spherical and range in size from 20-55 nm. The small misdeeds in particle's shape and size were due to the  uneven distribution of heat and mass during material ignition. 46 The results of TEM study and the diffraction examinations are in close alliance.

FTIR study
Fig. S1 † displays the FTIR spectral patterns of Y 3.97 Al 2 O 9 :0.03Tb 3+ nanophosphors recorded in 400-   4000 cm −1 . Band at 3446 cm −1 equivalent to -OH unit of water suggested the presence of moisture content in the synthesized materials. A weak band at ∼2364 cm −1 is assigned to the stretching vibrations of nitrate (-NO 3 ) unit. 47 The vibrational bands at ∼442 and 568 cm −1 are demonstrates the Al-O bond. 48 The bands present at 716 cm −1 and 791 cm −1 are assigned to Y-O bonds. 49 The results of FTIR study fairly corroborate with the XRD analysis data.

EDX investigation
The formation of pure Y 4 Al 2 O 9 :Tb 3+ nanophosphor has been conrmed via EDX spectral prole. The EDX prole (Fig. 7) evinced various peaks attributed to the elements present in the prepared lattice. EDX pattern of nano-phosphors consist peaks exclusively of yttrium, aluminum, oxygen and terbium suggesting the nonexistence of any additional element in the lattice and formation of desired lattice in proper stoichiometry. Inset of Fig. 7 demonstrates the eld view corresponding to the particular elements present in the synthesized optimum sample. The peculiar peaks of Tb 3+ cation conrmed the uniform doping of former ion in the synthesized lattice. Table 4 evinces the EDX data (atomic and weight %) of Y 3.97 Al 2 O 9 :0.03Tb 3+ nanophosphor. The outcomes of EDX specify the composition of homogeneous phosphor in desired stoichiometry with appropriate distribution of elements. These results are also found in accordance with structural and spectral data. Fig. 8 demonstrates the diffusion reectance spectra (DRS) spectra measured between 200 and 800 nm wavelength range which was used to determine the optical characteristics and energy band gap of undoped Y 4 Al 2 O 9 and optimized Y 3.97 Al 2 O 9 :0.03Tb 3+ nanosamples. From the gure, it is clear that there exist characteristic transitions of Tb 3+ ions located at 258 nm and 383 nm accredited with 4f 8 / 4f 7 5d 1 and 7 F 6 / 5 G 6 respectively. However, in the reectance spectrum of host lattice no such peaks were observed. Additionally, the Kubelka-Munk function (eqn (4)) may be employed to dene energy band gap of synthesized nanophosphors given as 50

Optical absorption analysis
Here, R is calculated reectance normalized with BaSO 4 and K & S represent the absorption and scattering coefficients, respectively. Though, relationship between band gap (E g ) and absorption-factor (a) of considered crystalline sample is determined through utilizing Tauc's relation (eqn (5)) Here, hn stands for photon energy. Combining eqn (4) and (5) gives an evaluation of the band gap energy values, which is then expressed as 51 Here, the hn refers to the photon energy,

Photoluminescence analysis
3.6.1 Excitation and emission spectrum of bulk material. Fig. 9(a) demonstrates the excitation spectrum of Y 3.97 Al 2 O 9 :0.03Tb 3+ bulk material. The excitation spectrum recorded at 548 nm, consists of one highly intense band at 249 nm from the 4f 8 / 4f 7 5d 1 transition of Tb 3+ ion and some sharp peaks from the 4f 8 / 4f 8 transition of Tb 3+ ions in the longer wavelength region located at 304, 327, 356, 377, 398 and 492 nm corresponding to the electronic transition from the 7 F 6 ground states to 5 H 6 , 5 H 7 , 5 L 9 , 5 G 6 , 5 L 10 and 5 D 4 respectively. 52 When the bulk Y 3.97 Al 2 O 9 :0.03Tb 3+ material was excited at a wavelength of 249 nm, the Tb 3+ ion (4f 8 ) would be raised to the higher 4 f 7 5d 1 level and would feed aerward to the 5 D 3 or 5 D 4 excited states. The PL spectra of bulk Y 3.97 Al 2 O 9 :0.03Tb 3+ material reveals several emission peaks at 416, 436, 458, 484, 543, 585 and 623 nm, which are attributed to the electronic transitions 5 D 3 / 7 F 5 , 5 D 3 / 7 F 4 , 5 D 3 / 7 F 3 , 5 D 4 / 7 F 6 , 5 D 4 /  Fig. 8(b). The emission spectrum lines can be separated in two groups. The blue emission group is from 5 D 3 / 7 F J (J = 5, 4 and 3) below 480 nm and the green emission group is 5 D 4 / 7 F J (J = 6, 5, 4 and 3) above 480 nm. From the Fig. 9, it is clear that the intensity of the excitation and emission spectra found to be high as compared to the optimized nanophosphors. The reason behind this high intensity is high temperature sintering (1100°C ) solid state reaction (SSR) method. Also, the excitation and emission spectra of bulk materials differ from the nanophosphors in respect of their shape and position of the peaks. Also, the loss of intensity in nanophosphor is due to crystalline defects and micro deformations acting as photoluminescence quenchers. 54 Furthermore, it has been reported that surface -OH groups are efficient quenchers when phosphors are synthesized by wet synthesis methods. 55 3.6.2 Excitation and emission spectra of nanophosphors. PLE behaviour of Y 3.97 Al 2 O 9 :0.03Tb 3+ nanophosphor was revealed in Fig. 10. The excitation spectrum has been examined at Tb 3+ emission of 548 nm, which mainly contains most intense and broad band at 258 nm derived from the 4f 8 / 4f 7 5d 1 transition of Tb 3+ along with excitation bands at ∼302 nm, 324 nm, 346 nm, 359 nm and 383 nm with 7 F 6 / 5 H 6 , 7 F 6 / 5 H 7 , 7 F 6 / 5 L 6 , 7 F 6 / 5 L 9 and 7 F 6 / 5 G 6 transitions, individually. 56 Among all the excitation bands, most intense excitation 4f 8 / 4f 7 5d 1 transition located at 258 nm was used to monitor the PL spectra of synthesized nanocrystalline sample. In the present synthesized nanocrystalline materials, there are four emission centres (Y1/Tb1, Y2/Tb2, Y3/Tb3 and Y4/Tb4) with two different coordinating sites (Y2/Tb2 and Y4/Tb4 are in six coordinated octahedral environment and Y1/Tb1 and Y3/ Tb3 are in seven coordination eld) are present. The PL characteristic of all the synthesized nanosamples viz. Y 4−x Al 2 O 9 :-xTb 3+ (x = 1-5 mol%) was noted at 258 nm excitation, as displayed in Fig. 11. Normally, the luminous behaviour of Tb 3+ is due to de-excitation from excited states ( 5 D 3 and 5 D 4 ) to the 7 F J=0-6 . Nanosamples get energized to the 4f 8 state on absorbing   photons of 258 nm wavelength and depopulated to the above mentioned excited states. Emission spectra entail different emissive peaks namely 5 D 3 / 7 F 5 (at 417 nm), 5 D 3 / 7 F 4 (at 439 nm), 5 D 4 / 7 F 6 (at 486 nm), 5 D 4 / 7 F 5 (at 548 nm), 5 D 4 / 7 F 4 (at 584 nm) and 5 D 4 / 7 F 3 (at 622 nm). PL spectra are divided into two regions which are known as blue region and green region. 57,58 The obtained emission bands i.e. at 417 nm ( 5 D 3 / 7 F 5 ) and at 439 nm ( 5 D 3 / 7 F 4 ) are belongs to the blue region of the spectra while the transition 5 D 4 / 7 F J (J = 3, 4, 5 and 6) related to the green section of the spectra. 59 Hyper-intense peak observed at 548 nm ( 5 D 4 / 7 F 5 ) was liable for green in synthesized nanosamples. Different energy levels of trivalent terbium ion are displayed in Fig. 12. 3.6.3 Concentration quenching mechanism. To analyze the concentration effect of Tb 3+ ion on the emission intensity of Y 4−x Al 2 O 9 :xTb 3+ (x = 1-5 mol%) nanophosphor, samples of numerous Tb 3+ ion content extending from 1 to 5 mol% have been fabricated. Fig. S3 † demonstrates the relation of emission intensity with respect to the concentration of Tb 3+ ion. This gure clearly revealed that initially the emission intensity increase and reached the maxima at 3 mol% Tb 3+ ion, and then it decrease up to 5 mol% of Tb 3+ ion concentration. This decrease in PL intensity at high concentration of Tb 3+ was the result of concentration quenching. This concentration quenching effect can be understands by the Dexter energy transfer mechanism. Commonly, two main aspects are responsible for the energy transfer between the neighbouring activator ions, one is exchange interaction (for which critical distance must be less than 5 Å) and second is multipolar interaction (for which critical distance should be greater than 10 Å). For the estimation of the critical distance (R c ), Blasse relation was used which is given by eqn (7) (ref. 60) Where, x C refers the critical concentration of Tb 3+ ions, V denotes cell volume and Z stands for a number of cations in a single unit-cell. For optimized sample, volume = 828.72 Å 3 and Z = 4 and the calculated R C value is 23.63 Å which validates that energy transfer takes place via multipolar interactions. To further authenticate the mechanism of energy transfer in Y 4−x Al 2 O 9 :xTb 3+ (x = 1-5 mol%) nanophosphors, Dexter's formula of multipolar interaction is used as follows eqn (8) log I The multipolar-interaction comprises quadruple-quadruple (M = 10), dipole-quadruple (M = 8), dipole-dipole (M = 6) and migration of energy among nearest ions (M = 3). 61 Correlation between log(x) vs. log(I/x) is obtained with linear t graph, as exhibited in Fig. 13. The obtained slope is −3.07, resulted into Q value of 9.21. This result suggesting that quadruple-quadruple interactions are answerable for concentration quenching (CQ) in prepared nanosamples.

Luminescence lifetime and quantum efficiency
The decay lifetime of Y 3.97 Al 2 O 9 :0.03Tb 3+ bulk and nanophosphor was monitored under an excitation on 249 nm (bulk, 4f 8 / 4f 7 5d 1 ) and 258 nm (nanophosphor, 4f 8 / 4f 7 5d 1 ) and emission at 543 nm (bulk, 5 D 4 / 7 F 5 ) and 548 nm (nanophosphor, 5 D 4 / 7 F 5 ) as demonstrates in Fig. 14(a) & (b). The decay curve is perfectly tted through second order exponential method and the formula is given below The above formulation incudes A 1 and A 2 which are the tting parameters, I 0 and I t (intensity at t = 0 and t respectively). s 1 & s 2 are the exp components of decay lifetimes, individually. s avg can be measured by the relation (10) as 62 The average lifetime values of Y 3.97 Al 2 O 9 :0.03Tb 3+ bulk and nanophosphor 2.481 ms and 2.831 ms respectively. The average decay lifetimes (s avg ) for all synthesized doped nanosamples are shown in Table 5. As we can observe, decay times decrease as the doping amount of trivalent terbium ion increases. As content of incoming ions rises, then ions move in closer proximity to one another and quickly transfer energy, providing different decay-path with reduced decay-lifetime. Fig. S4 † reveals that the s 0 value becomes 3.69 ms, obtained via Auzel's tting by the use of eqn (11) Where, s C represent decay lifetime, C 0 denotes constant value and N are number of phonon. Additionally, value of internal quantum efficiency (h) of activated nanosamples is intended by the ratio of average lifetime and radiative lifetime (eqn (12)). 63 h = s avg /s 0 Internal quantum efficiency (h) of selected nanophosphors is assessed to be 91.05, 84.56, 76.69, 72.89 and 66.93% for x = 1, 2, 3, 4 and 5 mol% respectively. These values suggest that the quantum efficiency continuous decreases with trivalent terbium concentration. Photoluminescence quantum efficiency is termed as the ratio of emissive photons to the absorbed photons. The external luminescence quantum yield of synthesized Y 4−x Al 2 O 9 :xTb 3+ (x = 1-5 mol%) nanophosphors was measured on a Fluorolog-3 Horiba Jobin Yvon equipped with a 150 W pulsed xenon lamp. External quantum efficiency measurements were done at room temperature at an excitation wavelength of 258 nm with a slit width of 1 nm for excitation and 1 nm for emission.
Fig. 16 claries color temperature values of prepared phosphors. All developed nanosamples revealed CCT values between 6500 and 9000 K, demonstrating that the light is produced as cold source. The color purity of Y 4−x Al 2 O 9 :xTb 3+ (x = 1-5 mol%) nanophosphors could be measured by eqn (15). 66,67 CP ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx À x i Þ 2 þ ðy À y i Þ 2 ðx d À x i Þ 2 þ ðy d À y i Þ 2 s Â 100 Here, (x d , y d ) and (x i , y i ) are the dominant and illuminated points separately. The color parameters for the chosen nanophosphors are listed in Table 5. The obtained Y 4−x Al 2 O 9 :xTb 3+ (x = 1-5 mol%) nanophosphor results can be viewed as a promising candidate for display applications and other future lighting foundations.

Conclusions
The current study concludes that less time consuming, urea assisted gel-combustion synthetic approach was used to synthesize the powdered samples of cool green color emanating Y 4−x Al 2 O 9 :xTb 3+ (x = 1-5 mol%) nanocrystalline materials. All cell characteristics determined via XRD aided Rietveld renement technique which is in excellent agreement with those found in the literature, and the addition of Tb 3+ had no discernible impact on the YAM phase structure. The TEM observation of powdered sample revealed particles with a diameter of less than 50 nm with agglomeration caused by coalescence, resemble with size predicted by Scherrer's equation utilizing XRD patterns. The synthesized nanomaterials found to be non-conducting in nature validates by their optical band gap values. The emission spectra recorded at 258 nm contains dominating band at 548 nm with 5 D 4 / 7 F 5 , responsible for green color in synthesized nanomaterials. The quadruple-quadruple interactions are accountable for the quenching of concentration in the prepared phosphors. The computed luminescent characteristics (quantum efficiency, CIE and CCT) for all nanosamples explore as potential candidates for the solid state lighting applications.

Data availability
Data will be made available on request.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this paper.